The invention relates to a method and apparatus for measuring the intensity of beams, in particular radioactive beams on surfaces.
For the measurement of levels of intensity of complex radioactive beams on surfaces such as chromatographs in analytical chemistry, it is nowadays conventional practice to use the technique of location-sensitive counters with positively charged counting wire.
In that procedure the particles which are emitted by the surface reach a gas-filled counting tube from below through an open entry window. Due to gas amplification, an electron avalanche is produced in the counting tube, the avalanche corresponding in respect of time and location to the position of the incident particles.
The counting pulses can be electromagnetically coupled into a delay line. The pulses pass along the delay line in both directions and the arrival of the pulses is electronically registered at each end of the delay line.
The time difference between the arrival of the two associated pulses, at the left and at the right, is a direct measurement in respect of the location of an incident particle.
The electron avalanche is delineated in respect of position by the voltage and the divergence of the particle beam from the point on the surface to be measured. While the inherent resolution, which is directly voltage-dependent, of the present day counting tubes, at a value of 0.2 mm, is sufficiently accurate, the degree of resolution in dependence on the particle beam with isotopes such as P-32, Cr-51, etc, with a resolution of several millimeters, is totally inadequate.
In order to achieve good spatial resolution, in accordance with present day techniques, the material to be measured is disposed as close as possible to the counting tube. However that spacing is subject to an absolute limit insofar as a minimum high tension must be maintained in the proportional counting tube between the testpiece and the counting tube.
In addition, the attempt is made nowadays to improve the level of resolution by electronic controls in respect of the pulse lengths. However the electronic control procedure causes losses in respect of the detectable particles (loss of efficiency), which, in the case of sources such as the frequently used H-3 and J-125, gives rise to measurement losses of 50% and more.